Project 1. Sustained, high rates of production of nitric oxide (NO) by cells of the immune system, as occurs during chronic inflammation, has been strongly implicated in the development of various forms of cancer. Quantifying pathophysiological levels of NO is crucial for understanding the relationship between endogenous NO synthesis and carcinogenesis. Equally important is knowing the concentrations of reactive intermediates derived from NO, such as nitrogen dioxide (NO2), nitrous anhydride (N2O3), and peroxynitrite (ONOO-), because these species are likely mediators of damage to cells arising from NO. It is not feasible to directly measure the concentrations of any of these compounds under most conditions of interest, yet such information is needed to correlate levels of toxicity and rates of mutation in cell cultures with actual levels of exposure, and to extrapolate those findings to situations in the body. There is a need also for